1. Field of the Invention
Aspects of the present invention relate to a method of preparing a supported catalyst, a supported catalyst prepared by the method, and a fuel cell using the supported catalyst, and in particular, to a method of preparing a supported catalyst by preparing a primary supported catalyst containing catalytic metal particles that are obtained by a primary gas phase reduction reaction of a portion of the final loading amount of a catalytic metal, and reducing the remaining portion of the catalytic metal by a secondary liquid phase reduction reaction using the primary supported catalyst, a supported catalyst prepared by the method, and a fuel cell using the supported catalyst.
2. Description of the Related Art
Fuel cells, which are considered to be a clean energy source of the future that can replace fossil energy, have high output densities and high energy conversion efficiencies, are operable at ambient temperature, and can be miniaturized and tightly sealed. Thus, fuel cells can be used in a wide range of applications, such as contamination-free automobiles, domestic power generating systems, mobile communication devices, medical instruments, military equipment, equipment for the space industry, and portable electronic instruments. Polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are electric power generating systems that produce direct current (DC) electricity from an electrochemical reaction involving methanol, water, and oxygen. Generally, these fuel cells have a structure comprising an anode and a cathode, to which a reaction liquid or gas is supplied, and a proton conducting membrane interposed between the anode and the cathode. The anode and the cathode contain catalysts, which degrade hydrogen or methanol to generate protons. The protons thus generated then pass through the proton conducting membrane and react with oxygen, the reaction being catalyzed by the catalyst present in the cathode, to produce electricity. Therefore, the significance of catalysts in such fuel cells is considerable.
Currently, in the case of PEMFC, both the anode and the cathode employ platinum (Pt) particles dispersed over the surface of an amorphous carbon support, and in the case of DMFC, the anode employs PtRu, while the cathode employs platinum in the form of metal particles or dispersed on an amorphous carbon support. However, using metal particles as the catalyst often results in much better reactivity, and thus, a supported metal catalyst is rarely used in DMFCs.
However, in the case of DMFC, the catalyst cost represents a large percentage of the total production cost, and thus it is necessary to reduce the amount of the catalyst used so as to secure a competitive price for the fuel cell. As an attempt to successfully reduce the amount of the catalyst used in the anode or the cathode, it has been suggested to use a carbon support which can increase the catalyst activity or increase the degree of dispersion of catalytic metal particles, compared with the amorphous carbon supports that are currently in use, or to use a supported catalyst which has a high concentration of catalytic metal particles dispersed on the support with a high degree of dispersion. Thus, it is necessary to develop a process for preparing such supported catalysts having high concentrations of catalytic metal particles at high degrees of dispersion, and it is also necessary to re-design the structure of the membrane electrode assembly (MEA) so as to obtain high performance, such as maximal catalyst activity.
Active research is being conducted to develop electrodes, fuel, and electrolyte membranes that are used in fuel cells, and to enhance the output density and the output voltage by increasing the energy density in fuel cells. In particular, there has been an attempt to enhance the catalyst activity of the catalysts used in fuel cell electrodes. The catalysts used in PEMFC or MDFC generally contain Pt or alloys of Pt with other metals, and thus it is necessary to reduce the amount of these catalytic metals used in order to secure competitive prices of the catalysts. Accordingly, in an attempt to reduce the amount of catalyst while maintaining or increasing the performance of a fuel cell, a method of increasing the specific surface area of a catalytic metal by using a conductive carbon material having a large specific surface area as a support, and dispersing fine particles of platinum or an alloy onto the conductive carbon material support, is being used.
As the effective specific surface area of a catalyst is increased, the catalyst activity is increased, and thus, in order to increase the effective specific surface area, the overall amount of the supported catalyst used can be increased. However, in this case, the amount of the carbon support being used will be also increased, along with the increase in the overall amount of catalyst, and the thickness of the fuel cell containing the supported catalyst will also be increased, thereby resulting in an increase in the internal resistance of the fuel cell. It is also difficult to produce an electrode containing an increased amount of supported catalyst. Therefore, it is essential to maintain constant the amount of the support used, while increasing the concentration of the catalytic metal to be supported. However, before preparing a supported catalyst having a high concentration of catalytic metal, it is necessary to achieve a high degree of dispersion of catalytic metal particles by preparing very fine particles. The supported platinum catalysts that are currently in use have a loading concentration of 20 to 30% by weight, and it is reported (E. Antolini et al., Materials Chemistry and Physics, 78, 563 (2003)) that in the case of commercial catalysts marketed by E-TEK, Inc., when the concentration of Pt metal particles in the supported catalyst is increased from 20% by weight to 60% by weight, the size of the Pt metal particles also increases by approximately four times. Thus, the effect of increasing the loading concentration cannot be fully utilized when such supported catalysts are actually used in fuel cells.
U.S. Pat. No. 5,068,161 discloses a method of preparing a supported catalyst containing a platinum alloy by a solvent reduction technique, in which an excess amount of water is used as a solvent to dissolve a catalytic metal precursor, hexachloroplatinic acid (H2PtCl6). Subsequently, formaldehyde is used as a reducing agent to reduce the catalytic metal precursor, and the resulting reduction product is filtered and dried in a vacuum.
However, this method involving solvent reduction is disadvantageous in that the size of the catalytic metal particles varies depending on the type of the reducing agent, and the size of the catalytic metal particles also becomes too large at high loading concentrations of 30% by weight or greater.
In another method of preparing a carbon-supported catalyst, a catalytic metal precursor is dissolved in an excess amount of a solvent, a carbon support is impregnated with the catalytic metal precursor, the solvent is removed by drying, and then the catalytic metal precursor loaded onto the carbon support is reduced using hydrogen gas (H. Wendt, Electrochim. Acta, 43, 3637 (1998)). According to this method, since the solvent is added in an excess amount, a concentration gradient is generated in the process of drying, and since the concentration gradient induces a capillary phenomenon, a discharge of the catalytic metal precursor may occur onto the pore surfaces of the carbon support. Also, there still remains a problem that the size of the catalytic metal particles increases as the loading concentration increases. Moreover, it is still necessary to correlate the performance of MEAs with the increased activity of such supported catalysts.